Engineered Buckle initiators and Mitigating Measures

Buckles are deliberately initiated by introducing initiation sites (triggers) along the pipeline route to ensure that the pipeline laterally buckles in a planned scenario in order to avoid the induced axial compressive forces concentrating in a particular site.

Buckle is initiated by either one of the following parameters; effective compressive force in the pipeline, out-of-straightness (OOS) features and lateral breakout resistance. Lateral Buckling breakout having the highest uncertainty (Bruton et al., 2007).

Potential localization which is related to inhomogeneity in pipeline as-laid configuration, pipe-soil interaction and temperature leads to longer feed-in length to the largest buckles and increases the susceptibility to buckling and fracture.

Figure 1‐10: Different Regions in a buckle (Kein et al.)

As the temperature in the pipeline increases the slip length will therefore continue to feed-into the buckle after the buckle has been developed (Kein et al.). The length of the slip zone depends on the available frictional resistance to oppose the feed-in. A virtual anchor is developed where there is sufficient frictional force to constrain the slip completely.

The post effective force will therefore change to take into effect the compressive forces into the buckle. This scenario for the post effective force for an isolated single buckle is showed in Figure 1‐11 according to the analysis conducted by JP Kenny group.

According to the paper (Bruton et al., 2007), if lateral buckles are initiated at regular intervals along the pipelines, the loads are effectively shared between the buckle sites.

Moreover, the shorter the spacing between buckle initiators, the lower is the probability of buckle forming at t each site as desired. Therefore, selecting appropriate and suitable spacing is the key but a challenging task in design.

Figure 1‐11: Post Effective force of a single isolated buckle (Kein et al.).

If the temperature is further increased after the post buckling, more pipe length will feed into the buckle and will increase the moment of the buckle (Kein et al.). This could lead to formation of more buckles along the pipeline.

If the buckles are spaced such that the distance between successive buckles is less than the total buckle length (Lo + 2Ls) of an isolated buckle, the feed-in is shared between the two buckles as shown in Figure 1‐12 (Kein et al.). This is known as expansion sharing (DNV-RP-F110, 2007).

Figure 1‐12: Illustration of expansion sharing with multiple buckles (Kein et al.).

The conventional techniques to avoid buckling have been to restrain the pipeline by trenching, burying and rock dumping. Alternatively, the thermally induced stress in the pipeline can as well be relieved with the use of inline expansion spools or mid-line expansion spools (Cheuk, 2007).

In spite of this, these methods are becoming less cost-effective as the operating temperatures and pressures are being required to increase further as the exploration moves into deeper waters where trenching and burying are not viable. Hence, the pipeline is left exposed on the seabed and allowed to buckle laterally.

In accordance with the recommended practice, DNV-RP-F110 (2007) if the response from the applied loads exceeds the pipe cross-sectional capacity, mitigating measures have to be introduced.

Apart from the conventional ways of preventing buckling there are number of improved mitigating measures that have been utilized in the industry during the past years.

The lateral buckling concept has been a design concept that aims to work with the induced expansion phenomenon rather than working against the induced stresses on the pipeline and some of the measures that have been used are as follows:

a) Sharing of expansion into adjacent buckles:

This can be achieved by the use of rock dumping at intermittent sections, with the aim to increase the restraint to axial movement in order to reduce the feed-in into isolated buckles that may be triggered by imperfection or trawl gear (DNV-RP-F110, 2007).

b) Mid-line Expansion spool:

This utilizes the mid-line spool to absorb the pipe expansion under operational temperature and pressure. Figure 1‐13 shows a mid-line expansion spool which was modeled in U configuration and imposed to thermal expansion at both ends. (Kein et al.).

Figure 1‐13: Example of Mid‐line expansion spool (Kein et al.)

c) Vertical Triggers/Sleepers:

This is a method that utilizes initial vertical imperfection (Out-of-straightness - OOS) to initiate a lateral buckle. Pipe sleepers pre-laid across the seabed is used to raise the pipeline off the seabed. This will create a vertical imperfection, OOS, which will initiate a buckle at this section. Figure 1‐14 illustrates buckles initiated by trigger. The buckle crown elevates the pipe above the seabed and causes a reduction in lateral friction resistance, and hence reduces uncertainties concerning lateral pipe-soil interactions.

Trigger/Sleeper lowers the critical buckling force as a result of reduction in lateral friction resistance. This allows for higher thermal feed-in into the buckle site, therefore increasing the buckle spacing and as a result reducing the number of buckle initiator required (Kein et al.).

Figure 1‐15: 3D view of Buckle initiation using Vertical trigger (Kein et al.)

Figure 1‐14: Buckle initiating using Vertical sleepers (Kein et al.)

d) Buckle Initiation using distributed Buoyancy or additional insulation coating:

Figure 1‐16 illustrates buckle initiation using distributed buoyancy. The distributed buoyancy is added to reduce the weight at the intermittent sections. As the critical buckling force is a function of pipeline weight, the added distributed buoyancy leads to buckle initiations as the weight reduces.

Figure 1‐16: Buckle initiation using distributed Buoyancy (Bruton et al., 2005)

e) Snake –Lay Configuration:

Figure 1‐17 present typical snake lay configuration. The concept of snake lay is to deliberately install horizontal lay imperfections to trigger a sufficient number of buckles at pre-determined locations along the pipeline so that the thermal expansion is distributed among a number of buckles rather than being concentrated at a few buckle sites (Rundsag et al., 2008).

Figure 1‐17: Snake Lay Configuration (Kein et al.)